It is known (J. Org. Chem. 51 1277-1282 (1986)) that guanosine can be prepared from 4-carboxamide-5-amino-1-ribofuranosyl imidazole by a process in three steps which involves condensation with carbodiimide derivatives, cyclization with PdO present and treatment with NH.sub.4 OH. This process is not attractive because it requires use of the toxic compound phosgene to prepare the carbodiimide derivatives, and because the cyclization and the subsequent treatment with NH.sub.4 OH take a very long time.
It is also known that the compound of formula I in which R is H can be prepared by a process in which the compound of formula II ##STR4## is first methylated to form the corresponding thiometyl compound which is then cyclized in alkaline medium (A. Yamazaki, Nucl. Acids. Res., 3, 1976, 251-259). This process has the drawback that the toxic and evil swelling methylmercaptane is formed as a by-product, and besides the yield is poor. It is reported in the article that cyclization of the compound II using the heavy metal salt HgO is not possible.